In a seat back frame for a vehicle, there are cases in which a frame in which a plate member is made to have a U-shaped, open cross-sectional configuration (the cross-sectional configuration has an open portion) is applied. In such a frame, in order to suppress the amount of bending deformation at the time of a rear collision, flat tube portions or reinforcing members are mounted to the vertical frame (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 11-32865).
However, in this conventional seat back frame for a vehicle, the problem arises that an increase in the number of parts leads to an increase in weight.